Battery life and battery size are two important features of battery powered devices. The goal of each battery charge is to perform “work”: powering functions and features that add value, with minimal loss to house-keeping and safety circuits.
When potentially harmful or hazardous conditions arise, the battery may need to be disconnected. To disconnect the battery without reducing battery energy, battery disconnect switches are typically located in series with the battery.
FIG. 1 shows a simplified schematic diagram of a conventional reverse blocking battery switch 100 made up of two N-channel Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) 108 and 109 (shown in FIG. 2). The N-channel MOSFETs include source inputs 101 and 106, and gate inputs 102 and 105.
The N-channel MOSFETs also include respective drains 103 and 104, which are connected by jumper 107. The two MOSFETs of the switch of FIG. 1 may be connected in a “common drain” configuration, utilizing an intrinsic body diode that inherently blocks the flow of current in one direction. Alternative configurations may be used, and other circuitry may be added to protect the various transistor inputs from static damage, or to level-shift the gate inputs 102 and 105.
FIG. 2 shows a perspective view of the mechanical construction of the conventional MOSFET, bi-directional switch configuration of FIG. 1. In this single package solution, common drains 103 and 104 are connected by die-attaching two standard MOSFETs 108 and 109 to a common copper die pad 107. Gates 102 and 105 are formed on the length-wise ends of the package. This configuration achieves both a direct thermal path for heat sinking, and a low electrical resistance connection between the drains 103 and 104, through the backside of vertical conduction MOSFETs 108 and 109.
FIG. 3 shows a simplified perspective view of a monolithic implementation of the conventional bi-directional switch of FIGS. 1-2. This configuration uses two MOSFETs built adjacent and joined to each other on the die pad 307. The MOSFETs are built commonly from two adjacent die 301 and 304 on a normal wafer layout, so the two die 301 and 304 are usually side-by-side, and the gate inputs 302 and 305 lie on the same end of the package. Although relatively easy to construct, the configuration of FIG. 3 does not fit a die package having an aspect ratio that is typically desired by manufacturers.
FIG. 4 shows a bi-directional switch in a 2×5 mm Dual Flat No Lead (DFN) package 400. This version of the bi-directional switch attempts to fit the die into a preferred package footprint. Since the aspect ratio of package 400 is 2×5 mm, the two MOSFETs are attached end-to-end.
However, this configuration renders the internal drain connection resistance high relative to the resistance of the vertical conduction MOSFETs. In order to make the series drain resistance tolerable, the backside of the die is die-attached to a copper die pad. This configuration allows for lower resistance by placing a copper plate (the die pad) in parallel with two bulk drain resistances. The die pad also serves as a common drain connection to external connectors.
While the above configurations are effective, there is a need in the art for a switch having improved characteristics.